This invention pertains to carbonation of cementitious materials, particularly to carbonation of cements using supercritical or high density carbon dioxide.
Above a compound""s xe2x80x9ccritical point,xe2x80x9d a critical pressure and temperature characteristic of that compound, the familiar transition between gas and liquid disappears, and the compound is said to be a xe2x80x9csupercritical fluid.xe2x80x9d Supercritical fluids (SCFs) have properties of both gasses and liquids, in addition to unique supercritical properties. A supercritical fluid is compressible like a gas, but typically has a density more like that of a liquid. Supercritical fluids have been used, for example, as solvents and as reaction media. The critical pressure and temperature for carbon dioxide are 1071 psi and 31.3xc2x0 C. The viscosity and molecular diffusivity of a supercritical fluid are typically intermediate between the corresponding values for the liquid and the gas. Compounds below, but near, the critical temperature and pressure are sometimes termed xe2x80x9cnear-critical.xe2x80x9d
Hardened or cured cements have sometimes been reacted with high pressure or supercritical CO2 to improve their properties. Supercritical and near-critical CO2 increase the mobility of water that is already present in the cement matrix, water bound as hydrates and adsorbed on pore walls. A pore in the cement may initially contain supercritical or near-critical CO2 at the pore entrance, a dispersed water phase associated with the pore walls, and possibly free water at the CO2/water interface. The high CO2 pressure increases the solubility of CO2 in the dispersed aqueous phase. A concentration gradient of CO2 is thus produced in the concrete pores. Carbon dioxide may then react with various cement components, particularly hydroxides of calcium. (As used in the specification and claims, the term xe2x80x9chydroxides of calciumxe2x80x9d includes not only Ca(OH)2, but also other calcareous hydrated cement components, e.g., calcium silicate hydrate.)
Densification Reactions
Carbonation reduces the permeability of cement, typically by 3 to 6 orders of magnitude. This reduction in permeability has been attributed to precipitation of carbonates in the micropores and macropores of the cement. For example, in cement grout carbonation shifts a bimodal pore distribution (pores around 2-10 nm in diameter and pores around 10-900 nm) to a unimodal distribution (pores around 2-10 nm in diameter only). Reduced permeability and smaller pore diameters slow rates of diffusion in carbonated cements. For example, Clxe2x88x92 and Ixe2x88x92 diffusion coefficients have been reported to be 2 to 3 orders of magnitude lower in carbonated cement than in noncarbonated cement, as have carbon-14 migration rates. (Lower Clxe2x88x92 and Ixe2x88x92 diffusion rates indicate greater resistance to salt intrusion. Salt intrusion is undesirable, as it can lead to fracturing or cracking.) Curing cement grout with carbon dioxide increases the strength and dimensional stability of a cement. The pH of cement in fully carbonated zones is lowered from a basic xcx9c13 to a more neutral value of xcx9c8, allowing the reinforcement of the cement with polymer fibers such as certain polyamides (e.g., nylons) that are incompatible with normal cements.
Carbonation of cement is a complex process. All calcium-bearing phases are susceptible to carbonation. For calcium hydroxide (portlandite) the reaction is
Ca(OH)2+CO2xe2x86x92CaCO3+H2O 
The calcium carbonate may crystallize in one of several forms, including calcite, aragonite and vaterite. Calcite is the most stable and common form.
In this reaction, calcium hydroxide (Ca(OH)2) is assumed first to dissolve in water, after which it reacts with CO2. Following reaction, the calcium carbonate (CaCO3) precipitates. Atmospheric concentrations of CO2 (xcx9c0.04%), do not react appreciably with completely dry concrete. Conversely, if the concrete pores are filled with water, carbonation at low pressure essentially stops before bulk carbonation of a thick cement form can occur, because the solubility and diffusivity of CO2 in water are low under such conditions. However, bulk carbonation of cement can occur at atmospheric pressure and ambient temperatures after years of exposure to atmospheric carbon dioxide.
High pressure conditions have previously been used to carbonate the surface layers of hardened cements. However, problems resulting from bulk carbonation of hardened cements have been reported. For example, the volume changes associated with conversion of calcium hydroxide to calcium carbonate have been reported to cause microcracking and shrinkage, at least under certain conditions.
Supercritical Fluids in Cementitious Materials
Supercritical and near-critical fluids confined in narrow pores have properties that are often quite different from those of a bulk gas. Because supercritical fluids are highly compressible, a surface or wall potential can produce a strong, temperature-dependent preferential adsorption, which might not occur at all at lower fluid densities. For example, a water layer on the solid surfaces is believed to be necessary to initiate carbonation reactions. Water is, in turn, a product of carbonation. At lower pressures water can completely fill the pores and thereby limit or even prevent carbonation; in such cases the sample must be dried for carbonation to resume. However, saturation and supersaturation of water in a CO2-rich phase is possible at high pressure, because phase separation in the concrete pores is slower than the carbonation reaction. Also, at high pressures carbon dioxide may adsorb onto the solid surfaces, along with water. The pore environment may eventually consist of a fluid phase of water and dissolved CO2, with mostly water but some CO2, adsorbed onto the walls of the concrete pores. At high pressures solubility of CO2 in water increases.
E. Reardon et al., xe2x80x9cHigh Pressure Carbonation of Cementitious Grout,xe2x80x9d Cement and Concrete Research, vol. 19, pp. 385-399 (1989) discloses treating a solid, hardened, cementitious grout with carbon dioxide gas at pressures up to 800 psi, and notes that this process can sometimes cause physical damage to specimens, including fracturing due to dehydration and shrinkage.
J. Bukowski et al., xe2x80x9cReactivity and Strength Development of CO2 Activated Non-Hydraulic Calcium Silicates, Cement and Concrete Research, vol. 9, pp. 57-68 (1979) discloses treating non-hydraulic calcium silicates with CO2 up to 815 psi, and notes that both the extent of the carbonation reaction and the compressive strength of the carbonated materials increased with treatment pressure.
U.S. Pat. No. 4,117,060 discloses a method for the manufacture of concrete, in which a mixture of a cement, an aggregate, a polymer, and water were compressed in a mold, and exposed to carbon dioxide gas in the mold prior to compression, so that the carbon dioxide reacts with the other ingredients to provide a hardened product.
U.S. Pat. No. 4,427,610 discloses a molding process for cementitious materials, wherein the molded but uncured object is conveyed to a curing chamber and exposed to ultracold CO2.
U.S. Pat. No. 5,518,540 discloses treating a cured cement with dense-phase gaseous or supercritical carbon dioxide. The patent also mentions using supercritical carbon dioxide as a solvent to infuse certain materials into a hardened cement paste. See also U.S. Pat. No. 5,650,562.
U.S. Pat. No. 5,051,217 discloses a continuous stamping and pressing process for curing and carbonating cementitious materials. CO2 was admitted at low pressures, and could later be compressed to higher pressures in one segment of the apparatus, a segment through which an afterhardening cement mixture passed continuously. The apparatus was said to be quasi-gas-tight. Only a portion of the uncured form was subjected to high pressure at any given time. The ratio of the mass of CO2 to the mass of the uncured cement was relatively low, apparently always under 0.002 (extrapolating from data given in the specification).
F. Knopf et al., xe2x80x9cDensification and pH Reduction in Cement Mixtures Using Supercritical CO2,xe2x80x9d Abstract of paper to be presented at 1997 annual meeting of the American Institute of Chemical Engineers, available on the Internet in July 1997 at
http://www1.che.ufl.edu/meeting/1997/annual/session/100//h/index.html
discloses some of the inventors"" own work, work that is disclosed in greater detail in the present specification.
We have discovered that a superior method to rapidly carbonate large cement forms or structures is to shape and harden the cement in a mold under high carbon dioxide pressure, at supercritical, near-supercritical, or high CO2 density conditions. In other words, contrary to previous teachings, supercritical, near-supercritical, or high density CO2 is reacted with cement while the cement is still in an uncured state. The novel carbonation method is more reliable, efficient, and effective than are post-molding treatments with high-pressure CO2, or treatments using low temperature, low pressure CO2. The novel method is more effective and reliable than methods that admit relatively small amounts of CO2 to a mold at relatively low pressure, and then compress the uncured mixture. The novel method is more effective in penetrating voids with CO2, and is therefore more efficient in converting hydroxides of calcium to CaCO3. Cements molded in the presence of high-pressure CO2 are significantly denser than otherwise comparable cements having no CO2 treatment, and are also significantly denser than otherwise comparable cements treated with CO2 after hardening.
The novel bulk carbonation of cementitious materials produces several beneficial effects, including reducing permeability of the cement, increasing its compressive strength, and reducing its pH. These effects are produced rapidly, and extend throughout the bulk of the cementxe2x80x94they are not limited to a surface layer, as are prior methods of post-hardening CO2 treatment. The novel method may be used with any cement or concrete composition, including those made with waste products such as fly ash or cement slag. Surface carbonation is almost instantaneous, and bulk carbonation is rapid even with forms several centimeters thick, tens of centimeters thick, or thicker. By combining molding, curing, and carbonation into a single step, carbon dioxide is better distributed throughout the entire specimen or form, producing a uniform carbonated cement product. In particular, it is believed that this is the first cured cement in which all interior portions of the cement that are at least 1 mm from the nearest surface of the cement comprise interlocking calcium carbonate crystals that are at least 10 xcexcm in diameter.
Bulk carbonation of cement with supercritical CO2 in our laboratory has produced a dense layer of interlocking calcium carbonate (calcite) crystals in minutes. The crystals are an order of magnitude larger in diameter (xcx9c10 xcexcm) than has been previously reported for calcite crystals in the interior of cements. The novel process produces concretes with improved durability and higher compressive strengths.
Uses for concretes based on the novel, bulk-carbonated cements are numerous. The higher compressive strength allows the use of thinner blocks and less material for a given strength requirement. For example, the stronger concrete may be used to make lighter weight, fire-resistant structural panels or roofing tiles. Cement roofing is rapidly gaining acceptance. These roofs last essentially for the lifetime of the home, have a Class A fire rating, and can be cast into any desired appearance. Costs should be competitive with those for shorter-lived asphalt roofing materials.
Low-cost reinforcing fibers may be used in bulk carbonated cements due to the near-neutral pH of these materials. Many potential reinforcing fibers are incompatible with the higher pH found in most cements, e.g. the pH xcx9c13 of conventional Portland cements. For example, it has been estimated that 3-4 billion pounds of carpet fiber per year are land-filled in the United States. Recycled carpet polymers could instead be used to reinforce these cement structures of near-neutral pH, transforming old carpets from a waste product into a useful resource.
Carbonated cementitious materials can also be used for building artificial reefs. Near-neutral pH""s are necessary for the growth of most marine organisms.
Carbonation and polymer reinforcement produce concretes with greater resistance to chemical attack, a property that is useful, for example, in the petroleum, mining, metallurgical, and chemical industries. Bulk-carbonated cements have essentially no die-swell or warpage, an advantage in the ceramics industry.
Preparation of Carbonated and Molded Samples
Comparison samples using previously cured cements were prepared in an existing SCF continuous treatment system. Liquid CO2 was compressed by a positive displacement diaphragm compressor (American Lewa model ELM-1) to 1500 psi. The compressed CO2 was stored in surge tanks to dampen pressure fluctuations. The pressure was controlled by a Tescom regulator (model 44-1124) to within xc2x15 psi. Pressure was monitored by a Heise digital pressure gauge (model 710A). The specimen (10 mm by 10 mm by 40 mm) was held in a tube immersed in a Plexiglas 25xc2x0 C. constant temperature bath. The CO2 flow rate was xcx9c0.8 g/s, and the run time was 1 hour.
A prototype device was constructed to evaluate the novel one-step method for molding, curing, and supercritical (or near-critical or high density) CO2 treatment. Specimens were treated in a simple cylindrical mold operated by a piston, which was sealed on its outer surface by O-rings. CO2 gas (at xcx9c700 psi) was introduced below the piston. The pressure above the piston was rapidly increased using water as a driver fluid. The increased pressure initiated the molding process. As the piston moved rapidly toward the sample, the gas pressure above the sample rose to equalize. But simultaneously the CO2 reacted with the cement, tending to lower the pressure. A 2000 psi water pressure was applied to the piston, and the samples were generally molded for xcx9c3 hours, although shorter or longer times can be used. The molded specimens in the prototype embodiment were cylindrical, 39 mm diameter by 13 mm height. The prototype unit allowed various modes of CO2 addition to be studied, without the complexities inherent in filling the mold with uncured cements under pressure. However, the scope of the invention is not limited by the manner used to fill the mold. The amount of CO2 added to the cement matrix could be readily controlled by adjusting the initial height of the piston above the cement.
Characterization of Chemical and Physical Properties of Cements
The porosities of conventionally cast samples (i.e., conventionally molded without high pressure CO2) and samples produced by the novel process were determined indirectly by measuring surface areas at a fixed initial composition. Higher surface areas are often associated with void-filling and therefore with decreased pore volumes, when small pores are created from larger pores without significant pore closure. The amount of nitrogen or other inert gas adsorbed (in determining surface area) includes contributions from capillary condensation in small pores. However, as voids are completely filled surface areas decrease significantly. A discussion of physical adsorption mechanisms in porous materials can be found in standard works on this subject, for example, D. M. Ruthven, Principles of Adsorption and Adsorption Processes (1984).
Thus an increase in surface area upon carbonation indicates a small reduction in voidage, while a decrease in surface area indicates almost complete closure of voids in the specimen, accompanied by densification. Surface areas were estimated using the one-point BET method at 30% relative saturation, using a Micromeritics 2700 Pulse Chemisorption apparatus. Water was first removed under vacuum at 1 torr for 24 h at ambient temperature, then under flowing N2/He for at least 2 h. The surface areas of selected samples were checked by the full BET N2 adsorption method using an Omnitherm (model Omnisorp 360) adsorption apparatus. The pore volume was determined in water by displacement (Archimedes"" principle). All specimens used in density and porosity measurements were dried under vacuum at 1 torr at ambient temperature prior to measurement.
A Scintag PAD-V automated X-ray Powder Diffractometer was used to identify crystalline phases. Specimens were step-scanned from 3-60xc2x0 2xcex8, at a 0.02xc2x0 step size, 3 second/step. A Perkin-Elmer thermogravimetric analyzer was used to quantify weight losses from water evolution (from hydrates), hydroxide (e.g., Ca(OH)2) to oxide (e.g., CaO) conversions, and carbonate (e.g., CaCO3) to oxide (e.g., CaO) conversions. The carrier gas was helium at 1 atm. The temperature program was 200-700xc2x0 C., 5xc2x0 C./min, hold at 700xc2x0 C.
Results, Post-Treated Samples
The xe2x80x9cpost-treatmentsxe2x80x9d (i.e., carbonations of previously cast samples) used near-critical CO2 (1500 psi and 25xc2x0 C.). The CO2 density at these conditions was 0.83 g/cm3, well above the density at the critical pressure and temperature (0.46 g/cm3). Table 1 summarizes X-ray diffraction (XRD) results for five different concrete mixes. The samples for the XRD measurements were taken from the surfaces of the specimens. For each mix both a control sample (no carbonation) and a test sample (carbonated) were measured. The reported weights of the additives were normalized to the initial weight of concrete. For all samples, a weight ratio of 0.603 water to 1.0 cement (ASTM Type III) was used in the initial mix. The five mixes represent typical fast set concretes, some of which included one or more of the following additives: glass fibers, Kevlar fibers, calcite, lime, and a plasticizer.